Semiconductor optical device

ABSTRACT

The semiconductor optical device according to one of the embodiments includes a substrate, a optical waveguide layer formed on the substrate as a MQW layer having a plurality of well and barrier layers. The semiconductor optical device also includes an optical absorbing layer formed on the substrate and adjacent to the optical waveguide layer so that an incident light having an incident wavelength λ LD  through the optical waveguide layer is guided into the optical absorbing layer. Successively formed thereon are a cladding layer and a pair of electrodes. Each of the well layers has a wavelength λg corresponding to a band gap thereof, that is greater than the incident wavelength λ LD . Also, a band gap energy between base levels of a conduction band and a valence band of the optical waveguide layer is larger than a photon energy of the incident light having the incident wavelength λ LD .

BACKGROUND OF THE INVENTION

[0001] 1) Technical Field of the Invention

[0002] The present invention relates to a semiconductor optical devicesuch as a semiconductor optical modulating device (semiconductor opticalmodulator) and a semiconductor optical receiving device (semiconductoroptical receiver), and in particular, to such a semiconductor opticaldevice used for an optical communication system of 1.3 μm-band.

[0003] 2) Description of Related Arts

[0004]FIG. 6 is a schematic perspective view of a conventionalsemiconductor light modulating device denoted by reference numeral 400.The semiconductor light modulating device 400 includes a substrate 1 ofInP, having an upper portion formed in a ridge configuration. Providedon the ridge portion are a light absorbing layer 2 and a pair of opticalwaveguide layers 13, so that the light absorbing layer 2 is sandwichedby the pair of the optical waveguide layers 13. The light absorbinglayer 2 may be formed as a bulk semiconductor layer or amulti-quantum-well layer (referred to as a “MQW layer”) of material suchas InGaAsP and InGaAlAs. On the other hand, the optical waveguide layer13 is formed as a bulk semiconductor layer of InGaAsP. Successivelydeposited on the light absorbing layer 2 and the optical waveguidelayers 13 are a cladding layer 4 of p-InP and a contact layer 5 ofp-InGaAsP. Further, deposited on a top surface of the contact layer 5and a bottom surface of the substrate 1 are an anode electrode 6 ofTi/Au and a cathode electrode 7, respectively.

[0005] In the semiconductor optical modulating device 400 of FIG. 6, anincident light 51, which is a continuous wave (referred to simply as“CW”) of 1.3 μm-band, enters the optical waveguide layer 13, and isguided into the light absorbing layer 2.

[0006] A reverse biasing voltage is applied between the anode andcathode electrodes 6, 7, which is in response to a modulating electricsignal of high frequency. To this result, the modulating electric signalcauses the light absorbing layer 2 to absorb the CW light due to theQuantum Confined Stark effect or the Franz-Keldysh effect.

[0007] Thus, the outgoing light 52 from the other optical waveguidelayer 13 has an amplitude and/or a phase modulated in the lightabsorbing layer 2.

[0008]FIG. 7 is an energy-band diagram of a region including andadjacent to the optical waveguide layer 13 in a cross section takenalong the line VI-VI of FIG. 6, illustrating the band gap of the opticalwaveguide layer 13 formed as the bulk semiconductor layer between thesubstrate 1 and the cladding layer 4. The optical waveguide layer 13 hasa composition ratio of InGaAsP selected so that it has a band gap energycorresponding to a photon energy of the light having a predeterminedwavelength (referred to simply as a “wavelength λg”), which is 1.1 μm orless, for example. The wavelength λg (1.1 μm) of the optical waveguidelayer 13 is set shorter than the wavelength of the CW light (1.3 μm) soas to reduce absorption of the CW light within the optical waveguidelayer 13.

[0009] However, in case where the optical waveguide layer 13 of InGaAsPhas the wavelength λg of 1.1 μm, it has the refractive index ofapproximately 3.30. Meanwhile, the substrate 1 of InP and the claddinglayer 4 of InP have refractive indices of approximately 3.21. Thus,there exists a small difference (0.09) of the refractive indices betweenthe substrate 1 and the cladding layer 4, and the optical waveguidelayer 13.

[0010] In order to confine the incident CW light in the opticalwaveguide layer 13 in an efficient manner as realized by a commerciallyavailable semiconductor modulating device of 1.55 μm-band, thedifference of the refractive indices therebetween should be 0.15 ormore.

[0011] Although the longer wavelength μg of the optical waveguide layer13 can improve the efficiency of the optical confinement, butdisadvantageously causes the optical waveguide layer 13 to absorb theincident light more.

SUMMARY OF THE INVENTION

[0012] Therefore, one of the embodiments of the present invention is toprovide a semiconductor optical device, in which the light of 1.3μm-band is less absorbed and confined in a more efficient manner in theoptical waveguide layer.

[0013] One of the embodiments of the present invention has an object toprovide a semiconductor optical device including a substrate, an opticalwaveguide layer formed on the substrate as a multi-quantum-well layerhaving well and barrier layers. The semiconductor optical device alsoincludes a light absorbing layer formed on the substrate and adjacent tothe optical waveguide layer so that an incident light having an incidentwavelength λ_(LD) through the optical waveguide layer is guided into thelight absorbing layer. It also has a cladding layer formed both on theoptical waveguide layer and the light absorbing layer, and a pair ofelectrodes formed so as to sandwich the optical absorbing layer. Each ofthe well layers has a wavelength λg longer than the incident wavelengthλ_(LD). Also, an effective transition energy gap between the lowestenergy levels of a conduction band and a valence band of the opticalwaveguide layer is larger than a photon energy of the incident light.

[0014] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter. Howeverit should be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the sprit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention more fully be understood from the detaileddescription given hereinafter and accompanying drawings which are givenby way of illustration only, and thus are not limitative of the presentinvention and wherein,

[0016]FIG. 1 is a schematic perspective view of a semiconductor opticaldevice according to Embodiment 1 of the present invention;

[0017]FIG. 2 is a band-energy diagram of a waveguide layer of thesemiconductor optical device according to Embodiment 1;

[0018]FIG. 3 is a graph illustrating a light modulating amplitudevarying in accordance with a thickness of the well layer;

[0019]FIG. 4 is a schematic perspective view of a Mach-Zender typesemiconductor optical modulating device according to Embodiment 2 of thepresent invention;

[0020]FIG. 5 is a schematic perspective view of a semiconductorwaveguide photodetector according to Embodiment 3 of the presentinvention;

[0021]FIG. 6 is a schematic perspective view of a conventionalsemiconductor optical device; and

[0022]FIG. 7 is a band-energy diagram of a waveguide layer of theconventional semiconductor optical device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Referring to the attached drawings, the details of embodimentsaccording to the present invention will be described hereinafter. Inthose descriptions, although the terminology indicating the directions(for example, “top”, “bottom”, and “upper”) are conveniently used justfor clarity, it should not be interpreted that those terminology limitthe scope of the present invention.

Embodiment 1

[0024] Referring to FIGS. 1 to 4, as one example of a semiconductoroptical device according to Embodiment 1 of the present invention, asemiconductor optical modulating device will be described hereinafter.FIG. 1 is a schematic perspective view of the semiconductor opticalmodulating device denoted by reference numeral 100, which is used formodulating light of 1.3 μm-band.

[0025] The semiconductor optical modulating device 100 includes asubstrate 1 of InP, having an upper portion formed in a ridgeconfiguration. Provided on the ridge portion of the substrate 1 are alight absorbing layer 2 and a pair of optical waveguide layers 3, sothat the light absorbing layer 2 is sandwiched by the pair of theoptical waveguide layers 3. The light absorbing layer 2 may be formed asa bulk semiconductor layer or a multi-quantum-well layer (referred to asa “MQW layer”) of material such as InGaAsP and InGaAlAs. Successivelydeposited on the light absorbing layer 2 and the optical waveguidelayers 3 are a cladding layer 4 of p-InP and a contact layer 5 ofp-InGaAsP. Further, deposited on a top surface of the contact layer 5and a bottom surface of the substrate 1 are an anode electrode 6 ofTi/Au and a cathode electrode 7, respectively.

[0026] According to the semiconductor optical modulating device 100 ofthe present embodiment, each of the optical waveguide layers 3 is formedas a MQW layer. FIG. 2 is an energy-band diagram of a region includingand adjacent to the optical waveguide layer 3 in a cross section takenalong the line I-I of FIG. 1.

[0027] Material composing the optical waveguide layer 3 is selected soas to confine an incident light in the optical waveguide layer 3effectively. In particular, the optical waveguide layer 3 is designed toinclude layers 31 of InGaAsP and barrier layers 32 of InP, which areformed alternately. The refractive index of InGaAsP can be controlled to3.53 or more by adjusting the composition (composition ratio) thereof.Thus, in case where each of the well layers 31 and barrier layers 32have thicknesses of approximately 6 nm, the effective refractive indexof the optical waveguide layer 3 can be set to 3.37 or more. Meanwhile,the substrate 1 of InP and the cladding layer 4 of InP sandwiching theoptical waveguide layer 3 have the refractive indices of 3.21.

[0028] Therefore, the optical waveguide layer 3 can be designed to havethe effective refractive index greater than those of the InP layers 1, 4by the difference of 0.16 or more so as to achieve an improvedefficiency in confining the light therein.

[0029] However, the well layer 31 of InGaAsP of which refractive indexis controlled to 3.53 or more has the wavelength λg that is longer thanthe wavelength λ_(LD) (=1.3 μm) of the incident light to be modulated inthe semiconductor light modulating device 100 (referred to simply as“λ_(LD)”). To this end, the light is absorbed by the well layer 31 ofthe optical waveguide layer 3.

[0030] In FIG. 2, the lowest energy levels of a conduction band and avalence band of the optical waveguide layer 3 are indicated by a pair ofimaginary lines 33, 34, respectively. Also, a band gap energy Eg_(MQW)of the optical waveguide layer 3 is defined between the base levels 33,34, which can be adjusted by controlling the thicknesses of the welllayer 31 and the barrier layer 32.

[0031] Therefore, Eg_(MQW) of the optical waveguide layer 3 can becontrolled greater than that a photon energy (Eg_(LD)=hc/λ_(LD)) of theincident light so that the light absorption in the optical waveguidelayer 3 is prevented. In other words, the thickness of the well layer 31can be appropriately adjusted so that the light absorption in theoptical waveguide layer 3 is prevented. To this result, the incidentlight of λ_(LD) (=1.3 μm) to be guided through the optical waveguidelayer 3 is well confined and not absorbed in the optical waveguide layer3.

[0032] Also, although needless to mention, the barrier layer 32 has theband gap energy which is transparent for the incident wavelength λ_(LD).

[0033] Next, referring to FIG. 3, the details will be described withrespect to the thicknesses of the well layer 31 and the barrier layer 32composing the optical waveguide layer 3.

[0034]FIG. 3 is a graph illustrating an extinction ratio varying inaccordance with the thickness of the well layer 3, when a voltage isapplied to the semiconductor light modulating device 100 including theoptical waveguide layer 3 formed as a MQW layer having a plurality ofwell layers 31 of InGaAsP and barrier layers 32 of InP. The incidentlight 51 to be modulated has the incident wavelength λ_(LD) of 1.3 μm,and the voltage applied between the anode and cathode electrodes 6, 7 is0-3 Volts to modulate the incident light.

[0035] As clearly shown in FIG. 3, the light is modulated when thethickness of the well layer 31 falls within the range of about 6.8 nm to9.5 nm, and the extinction ratio is maximum at the thickness of about8.3 nm. It is understood that the Optical Confined Stark effect isefficiently appeared for the thickness defined in the above range.

[0036] When the voltage is applied between the anode and cathodeelectrodes 6, 7 of the semiconductor optical modulating device 100, eachof the optical waveguide layers 3 has a portion adjacent to the lightabsorbing layer 2, to which the voltage is also applied. The lightabsorbing layer 2 is formed as a MQW layer, the light is absorbed by theportions of the optical waveguide layers 3 due to the Quantum ConfinedStark effect. This optical absorption makes a noise so that the qualityand/or reliability of the modulated signal of the semiconductor lightmodulating device 100 are deteriorated.

[0037] According to the semiconductor optical modulating device 100 ofthe present embodiment, each of the well layers 31 of the opticalwaveguide layers 3 has the thickness of 6 nm or less in order toeliminate the deterioration of the modulated signal.

[0038] Also, it should be noted that the thickness of the well layer 31is set to be 3 nm or more so that the light is effectively confined inthe optical waveguide layer 3.

[0039] Meanwhile, when the barrier layer 32 is too thin, the well layers31 sandwiching the barrier layer 32 have wave functions of electrons,which are coupled to each other. Then, the base energy level of the welllayers 31 is lower than that where each well layer 31 is isolatedsufficiently by the thick barrier layer 32. In other words, when theband gap energy of the optical waveguide layer 3 is less, the light of1.3 μm is absorbed thereby more.

[0040] In particular, it is well known that when the barrier layer 32 isthinner than approximately 5 μm, the light absorption by the opticalwaveguide layer 3 is increased. Thus, the thickness of the barrier layer32 should be approximately 5 μm or more.

[0041] As above, since each well layer 31 is set to have the thicknessof 6 μm or less, if each barrier layer 32 has the thickness of 6 μm ormore, then the optical waveguide layer 3 including the well layers 31and the barrier layers 32 would have the effective refractive indexlower, so that the optical confinement efficiency is reduced. Therefore,the thickness of the barrier layer 32 is preferably set to approximately6 μm or less.

[0042] Next, the operation of the semiconductor light modulating device100 will be described hereinafter. The incident light 51 having thewavelength of 1.3 μm-band (about 1.2 μm to 1.4 μm) enters the opticalwaveguide layer 3, and in turn is guided through the light absorbinglayer 2.

[0043] The reverse biasing voltage of 0-3 Volts is applied between theanode and cathode electrodes 6, 7, which is in response to themodulating electric signal of high frequency. To this result, themodulating electric signal causes the light absorbing layer 2 to absorbthe light guided through the light absorbing layer 2 due to the QuantumConfined Stark effect or the Franz-Keldysh effect.

[0044] Thus, the, amplitude and/or the phase of the light 52 outgoingfrom the other optical waveguide layer 3 are modulated.

[0045] Each of the optical waveguide layers 3 is formed as the MWQ layerof material described above, so that the light absorption by the opticalwaveguide layer 3 can be prevented and the optical confinement can berealized effectively.

[0046] Also, each of the well layers 31 and the barrier layers 32 aredesigned to have the predetermined thicknesses so that the noisegenerated in the optical waveguide layer 3 can be eliminated whileapplying the modulating electric signals.

[0047] In the above description, although the substrate 1 is made of InPand the well and barrier layers 31, 32 of the optical waveguide layer 3are made of InGaAsP and InP, respectively, they are not limited thereto,they can be made other materials described in the following table (Table1). TABLE 1 Material for Well Layer Material for Barrier Layer(composition ratio for (composition ratio for No. substrate satisfyingλg > λ_(LD)) satisfying λg < λ_(LD)) 1 InP InGaAlAs InP 2 InP InGaAlPInGaP 3 InP InGaAlAs InGaP 4 InP InGaAlAsSb InP 5 InP InGaAlAsSb InGaP 6InP InGaAsP InGaAsP 7 TnP InGaAlAs InGaAlAs 8 InP InGaAsP InGaAlAs 9 InPInGaAlAs InGaAsP 10 GaAs InGaNAs GaAlAs 11 GaAs InGaNAs GaAs 12 GaAsInGaAs GaAs

[0048] Further, materials indicated in Table 1 can be used for anoptical waveguide layer of a Mach-Zender type semiconductor lightmodulating device 200 according to Embodiments 2 and a Waveguide typesemiconductor light receiving devices 300 according to Embodiments 3, aswill be described hereinafter.

Embodiment 2

[0049]FIG. 4 is a schematic perspective view of a Mach-Zender typesemiconductor light modulating device denoted by reference numeral 200.Similar or the same components in FIG. 4 are denoted as similar or thesame reference numerals indicated in FIG. 1.

[0050] Similar to the semiconductor light modulating device 100according to Embodiment 1, the semiconductor light modulating device 200according to Embodiment 2 includes a substrate 1 of InP, including anupper ridge portion having a pair of branching members which arebranched and merged again. Provided on each of the branching members ofthe ridge portion are a first and second waveguide members 10, 20,including a pair of light absorbing layers 12, 22, each of which issandwiched by a pair of light waveguide layers 3. The light absorbinglayers 12, 22 and the optical waveguide layers 3 are made of materialsas described above. Also, each of the first and second waveguide members10, 20 includes a cladding layer 4, a contact layer 5, an anodeelectrode 6, and a cathode electrode 7, as similar to that of Embodiment1.

[0051] The incident light 51 of 1.3 μm-band enters the semiconductorlight modulating device 200 and is branched and guided into the opticalwaveguide layers 3 of the first and second waveguide members 10, 20.Then, the branched lights are merged again to the light 52 outgoing fromthe other optical waveguide layer 13. The optical paths running throughthe optical waveguide layers 3 of the first and second waveguide members10, 20 are designed to be substantially the same.

[0052] According to the Mach-Zender type semiconductor light modulatingdevice 200, a pair of reverse biasing voltages different from each otherare applied between the anode electrode 16 of the first waveguidemembers 10 and the cathode electrode 7, and between the anode electrode26 of the second waveguide members 20 and the cathode electrode 7. Thus,the light through the optical absorbing layers 12, 22 are modulatedindependently by the separate biasing voltages. Eventually, theindependently modulated lights interfere each other when combined. Thus,in the Mach-Zender type semiconductor light modulating device, theamplitude of the incident light 51 can be modulated to obtain theoutgoing light 52 by applying different voltages to the anode electrodes16, 26.

[0053] The optical waveguide layer 3 as described in Embodiment 1 can beused in the Mach-Zender type semiconductor light modulating device 200of the present embodiment so that the light absorption can be preventedand the optical confinement efficiency can be improved in the opticalwaveguide layer 3.

[0054] Also, when the electric modulating signals are applied to theanode electrodes 16, 26, the noises generated by the optical waveguidelayers 3 can be eliminated.

[0055] Therefore, the Mach-Zender type semiconductor light modulatingdevice 200 having a reduced optical loss and an improved modulationefficiency can be realized according to the present embodiment.

Embodiment 3

[0056] Referring to FIG. 5, as an another example of a semiconductoroptical device according to Embodiment 3 of the present invention, aWaveguide type semiconductor light receiving device will be describedhereinafter. FIG. 5 is a schematic perspective view of the Waveguidetype semiconductor light receiving device denoted by reference numeral300. Similar or the same components in FIG. 5 are denoted as similar orthe same reference numerals indicated in FIG. 1.

[0057] The Waveguide type semiconductor light receiving device 300 ofEmbodiment 3 has a structure similar to that of Embodiment 1 except thatthe optical waveguide layer 3 is provided on only one side (incidentside) of the light absorbing layer 2, instead of on both sides thereofas clearly shown in FIGS. 1 and 5.

[0058] In the Waveguide type semiconductor light receiving device 300,the incident light 51 comes in the optical waveguide layer 3 and isguided into the optical absorbing layer 2, in which the modulated lightis absorbed to generate a pair of a electron and hole. Thus, theWaveguide type semiconductor light receiving device converts theincident modulated light 51 to a current or electric signals in theoptical absorbing layer 2 varying in response to the varying amplitudeof the incident light 51. It should also be noted that the incidentlight used for the Waveguide type semiconductor light receiving deviceis of 1.3 μm-band.

[0059] The optical waveguide layer 3 as described in Embodiment 1 can beused in the Waveguide type semiconductor light receiving device 300 ofthe present embodiment so that the light absorption can be prevented andthe light confinement effect can be improved in the optical waveguidelayer 3.

[0060] Therefore, the semiconductor light modulating device 300 having areduced optical loss can be realized according to the presentembodiment.

[0061] As well illustrated in the aforementioned description, thesemiconductor optical device having the optical waveguide layer 3 formedas the MQW layer can reduce its optical loss, and achieve a highmodulating efficiency and a high detecting efficiency according to thepresent invention.

What is claimed is:
 1. A semiconductor optical device, comprising: asubstrate; an optical waveguide layer formed on said substrate as amulti-quantum-well layer having a plurality of well and barrier layers;an optical absorbing layer formed on said substrate and adjacent to saidoptical waveguide layer so that an incident light having an incidentwavelength λ_(LD) through said optical waveguide layer is guided intosaid optical absorbing layer; a cladding layer formed both on saidoptical waveguide layer and said optical absorbing layer; and a pair ofelectrodes formed so as to sandwich said optical absorbing layer;wherein each of the well layers has a wavelength λg corresponding to aband gap energy thereof, that is longer than the incident wavelengthλ_(LD); and wherein a band gap energy between base levels of aconduction band and a valence band of said optical waveguide layer isgreater than a photon energy of the incident light.
 2. The semiconductoroptical device according to claim 1, wherein each of the well andbarrier layers have the thicknesses controlled so that the differencebetween an effective refractive index of said optical waveguide layerand a cladding refractive index of said cladding layer is about 0.15 ormore.
 3. The semiconductor optical device according to claim 1, whereinthe wavelenth λ_(LD) falls within a range of about 1.2 μm and 1.4 μm. 4.The semiconductor optical device according to claim 1, wherein saidsubstrate and said cladding layer are made of InP.
 5. The semiconductoroptical device according to claim 1, wherein each of the well layers hasthe thickness of about 6 nm, and each of the barrier layers has thethickness in a range of about 5 nm and 6 nm.
 6. The semiconductoroptical device according to claim 1, wherein a combination of materialscomposing the well/barrier layers of said optical waveguide layer isselected from a group consisting of InGaAlAs/InP, InGaAsP/InGaP,InGaAlAs/InGaP, InGaAlAsSb/InP, InGaAlAsSb/InGaP, InGaAsP/InGaAsP,InGaAlAs/InGaAlAs, InGaAsP/InGaAlAs, and InGaAlAs/InGaAsP, respectively.7. The semiconductor optical device according to claim 1, wherein saidsubstrate is made of GaAs, and wherein a combination of materialscomposing the well/barrier layers of said optical waveguide layer isselected from a group consisting of InGaNAs/GaAlAs, InGaNAs/GaAs, andInGaAs/GaAs, respectively.
 8. The semiconductor optical device accordingto claim 1, wherein the incident light through said optical waveguidelayer and guided into said optical absorbing layer is modulated inresponse to voltages applied between said pair of electrodes.
 9. Thesemiconductor optical device according to claim 1, wherein the incidentlight through said optical waveguide layer and guided into said opticalabsorbing layer is converted in said optical absorbing layer to acurrent detectable from said pair of electrodes.